It is conventional practice to strap cargo onto pallets and to transfer the loaded pallets to or from storage vans or trailers by the use of forklift trucks. Wherever possible the forklift trucks are driven directly into the van to remove or deposit the loaded pallets from/on the floor of the van. However, where there are no loading docks or ramps it is usually not possible to drive the forklift truck into the van. In such instances it is often necesssary to use a truck equipped with an extendable boom that is mounted in cantilever fashion at the rear end of the truck; the boom carries a cargo-lift mechanism at its outboard or forward end. In use of the apparatus the truck is driven as close as possible to the rear end of the van so that the boom projects into the van interior. The forwardly-facing driver observes the position of the lift mechanism relative to the cargo, and accordingly operates suitable controls to raise or lower the boom for lifting or lowering palleted cargo. By extending or retracting the boom in the direction of its length the driver can transfer the cargo lengthwise of the van without relocating the truck.
In use of this boom-equipped truck a mechanical overloading problem exists because of the relatively great length of the boom and the location of the cargo-lift mechanism at the extreme outboard end of the boom. The boom is often comprised of a series of telescoped cylinder-piston assemblies of progressively decreasing diameter measured from the mounted end of the boom; hydraulic pressure is used to extend or retract the piston-cylinder assemblies. When relatively heavy cargo is being handled the boom sometimes has difficulty in raising the cargo from the van floor. Bending forces on the boom tend to produce a binding action between the piston and cylinder surfaces, thereby increasing hydraulic pressure requirements.
A principal aim of the present invention is to provide the cargo-lift mechanism with a lowerable roller mechanism that can forcibly engage the floor of the van to slightly elevate the cargo above the floor surface. The weight of the cargo is transferred from the outboard end of the boom to the floor of the van, whereby the boom is required only to push or pull the cargo lengthwise of the van, at least until the boom is retracted or shortened sufficiently to locate the cargo near the mounted end of the boom where it can be easily handled without producing an excessive force moment on the boom.
By using the lowerable roller mechanism it should be possible to achieve operating advantages in such respects as longer boom length for use in longer vans, a lighter boom structure, lower hydraulic pressure requirement or hydraulic pump volume requirement, smaller hydraulic reservoir requirement, longer operating life, and/or faster piston motion and consequent reduction in cargo manipulation time.